1. Field of the Invention
The present invention relates to a system for controlling adaptation layer 1 (AAL1) cell bandwidth in an asynchronous transfer mode (ATM).
In recent years, increasing use has been made of networks using ATM for providing broadband integrated service digital networks (BISDN).
Work on establishing standards for the ATM system is being performed at the International Telecommunications Union Telecommunication Sector (ITU-T). Protocol for the user network interface (UNI) (UNI protocol) has been recommended. In this recommendation, layers for open system interconnection (OSI) are defined. The ATM layer handles cell multiplexing and switching and defines the bearer capability of cells common for all applications. All information is transferred in cell units, so at the ATM layer handling the transfer of cells and lower layers, no processing is performed bearing in mind voice, data, video, and other media services. The quality of service required for each service (delay time, error rate, etc.) differs for each medium, however. Therefore, when converting the original information of the services into cells, an ATM adaptation layer (AAL) is provided for absorbing the differences in the quality conditions.
This AAL is comprised of a segmentation and reassembly sublayer (SAR) for segmenting information into cells and reassembling them and a convergence sublayer (CS) for guarantee against fluctuation of cells due to delay in accordance with the conditions required by the quality of service, restoration of the transmitting side clock frequency at the receiving side, error control for frames, and flow control. A number of types of AALs have been defined in accordance with the timing between the transmitting and receiving ends, the bit rate, and the parameters of the connection mode. Among them, AAL Type 1 (AAL1) specifications (e.g., format) have been defined for AAL Service Class A for providing a constant bit rate (CBR) such as voice or existing dedicated line services.
In AAL1, an SAR protocol data unit (SAR-PDU) to be added to an ATM header (5 bytes) is defined. An SAR-PDU is comprised of an SAR header (1 byte) and an SAR-PDU payload (47 bytes). A CS indication (CSI) bit is placed at the head of the SAR header. When transferring data by structured data transfer (SDT), that is, transfer of data having a frame structure such as 64 Kbps×n (n≧2), using the CSI bit, a pointer showing a boundary between frame data (lead position of frame) is set at the lead 1 byte of the SAR-PDU payload.
When transferring a consecutive data stream having such a frame structure converted into cells by the SDT mode of AAL1 (AAL Type 1 cells), if cells are discarded or erroneously added or there is bit error in the SAR header, the data rate between the transmitting and receiving ends becomes unstable. It is desirable to make improvements to stabilize this.
2. Description of the Related Art
As will be explained in detail later using the drawings, in an example of the related art, when only one cell is invalid, cells can be received by the reassembly buffer (RBUF). Further, when there is multiple bit error including the CSI bit or when using dummy cells to complement original P-format cells, the payload data of for example 46 bytes is reassembled into 47-byte data. By this, the data length thereof becomes different from the data length of the original data. Therefore, the data rates between the transmitting and receiving ends do not match and a gap arises in the data in a frame until a cell showing the next boundary (frame) arrives and therefore there is the problem that the frame format structure is lost.